audio_core: Preserve front channel volume after 6 to 2 downmix

Many games report 6 channel output while only providing data for 2. We only output 2-channel audio regardless, and in the downmixing, front left/right only provide 36% of their volume. This is done assuming all of the other channels also contain valid data, but in many games they don't. This PR alters the downmixing to preserve front left/right, so volume is not lost.

This improves volume in Link's Awakening, New Super Mario Bros U, Disgaea 6, Super Kirby Clash.